The present invention relates to systems for controlling the air-fuel ratio for an internal combustion engine, particularly an internal combustion engine for motor cars.
A feedback device for controlling the air-fuel ratio of an internal combustion engine by using a signal from an oxygen sensor (O.sub.2 sensor) which is located adjacent to a three-way catalytic converter in the exhaust system has been known. The feedback control is carried out in such a manner that the amount of air which enters a carburetor is controlled, or the amount of fuel or air which enters the intake manifold of the engine is controlled. In either case, when the oxygen sensor produces a "RICH" signal, the control is carried out so as to make the air-fuel ratio "LEAN", and when the oxygen sensor produces a "LEAN" signal, the control is carried out so as to make the air-fuel ratio "RICH". Thus the air-fuel ratio is controlled to a desired value, and hence the three-way catalytic converter is operated in a range where high cleaning efficiency can be achieved.
However, the above-described, known method creates a problem which is best explained with reference to the wave forms illustrated in FIG. 1. That is, when the running state of the engine changes at t.sub.1 from the high power state to the idle state, the actual air supply to the intake manifold of the engine changes along the SLOPE-1 in waveform (IV) of FIG. 1, but the air supply which is actually required by the engine is expressed in waveform the broken line SLOPE-2 in (IV) of FIG. 1. This results from the fact that the control voltage V.sub.f produced in the feedback control device changes rather slowly in accordance with the integration characteristic of the feedback control device. Accordingly, the resulting condition of excessive air supply may cause a situation wherein the ignition of the air-fuel mixture by engine is not carried out so that the engine stalls or surges.
It is, therefore, an object of this invention to provide an improved device for controlling the air-fuel ratio for an internal combustion engine which is not subject to the foregoing and other problems in the prior art.